Skin is constantly exposed to an aggressive environment. To examine the effect of ultraviolet irradiation (UVA) on protein damage in skin, HaCaT- cells (an immortal cell-line of human keratinocytes) were exposed to UVA, and then the level of protein carbonyl groups (a measure of protein damage) was determined by conversion of the carbonyl groups to their 2,4- dinitrophenylhydrazone or dansylhydrazone derivatives. Exposure of the cells to a single dose of UVA led to a substantial increase in the protein carbonyl content. A similar increase in carbonyl content was obtained by exposure of the cells to a non-cytotoxic concentration (0.1 mM/ml medium) of hydrogen peroxide. During three hours of recovery time following a single dose of UVA, 50% of the oxidized proteins disappeared, suggesting that the cells possess a protease capable of degrading oxidized proteins; however, efforts to demonstrate the presence of such a protease have been unsuccessful. The carbonyl content of human stratum corneum was found to increase with subject age. The carbonyl content of the horny layer increases linearly up to 50 years and then increases exponentially as a function of age over the range of 50-70 years.